Photoconductive image intensifier



March l, 1960 B, KAZAN PHo'rocoNDucTIvE IMAGE INTENSIFIER Filed Nv. 25.1955 INVENTOR. 55mm/HN KHZHN TOR/V57 5m'. au

United States Patent O PHOTOCONDUCTIVE IMAGE INTENSIFIER 4BenjaminKazan, Princeton, NJ., assignor to 'Radio Corporation of America, acorporation of Delaware Application November 25, 1955, Serial No;549,038

Claims. (Cl. 313-65) This invention relates to photoconductive imageintensiier devices, and particularly to Vdevices that will in- `tensfy-visible images, and will intensify and make visible infrared images orX-ray images.

There are several known types of devices that intensify images by meansof photoemissive effects. As is known, currents produced byphotoconductive action are rela- ,tively high as compared to currentsproduced by photoemissive action. Therefore, it is desirable to utilizedevices having photoconductors as the photosensitive element in orderthat the device will respond to weak signals. iln devices prior to thisinvention that have used .photoconductors as the sensitive element, theinformation has normally been obtained by scanning the rphotoconductorwith an electron beam, and obtaining the majority of the gain inamplifying systems outside of the tube.

It is therefore an object of this invention to provide an improvedphotoconductive intensiiier device.

It is another object of this invention to provide an improvedphotoconductive intensiiier device that is ,extremely sensitive and iscapable of visibly reproducing weak images of the visible or invisibleranges of the spectrum.

'I'hese and other objects are accomplished in accordance with thisinvention by providing a novel image intensifying device which includes,within an evacuated envelope, a target electrode comprising aphotoconductive layer and a phosphor layer superimposed, one on theother. A stream of electrons is directed onto the phos- .phor at such alow energy that it produces little or no light when no images aredirected onto the photocon- ,ducton When an image is focused onto thephotoconductive layer, the photoconductor becomes more conductive and acharge is established on the phosphor so that the landing velocity ofthe stream of electrons is increased, in a pattern corresponding to theimage whereby the phosphor emits light in the areas of the focusedimage. This emitted light is visible and corresponds to the originalimage. The original image may he visible, infrared or X-ray, and thereproduced image vis of an intensity that is much greater than theoriginal image.

The invention will now be described in detail in conjunction with theaccompanying single sheet ofdrawings wherein:

Figure 1 .is a transverse sectional view of a photocon- .dilutive imageintensifying device in accordance with this invention;

Figure 2 is :an enlarged fragmentary sectional view of .an embodiment ofthe target structureshown in Figure l; and,

Eigure 3 is a transverse sectional View ,of an embodiment .of .aphotoconductive image intensifying device in accordance with thisinvention.

Referring now to Figure 1 indetail, there is shown a transversesectional view of a photoconductive VVimage intensifying device, ortube, in Aaccordance with this invention. vThe tube '10.comprisesanevacuated envelope 11. images are directed ontothe envelope lthrough awall FFice 15 and are viewed through a Wall '13. Arranged in :an oiaxisrelationship to the envelope 11 is a neck portion 17 that encloses anelectron gun 19. The electron lgun 19 may be any conventional type ,andincludes a cathode 21, a control electrode 20, and one or moreaccelerating electrodes 22. The gun 19 is of the type that isy adaptedto produce a wide angle stream of electrons 23.

Arranged within the envelope Y11, inthe path of `the stream of electrons23, and adjacent to the image input wall I5, ofthe envelope 11, is atarget electrode 2'5. The target electrode 25 includes a transparent,support yplate '27 which may be made of a material such `as glass.'When desired, the target 25 may be supported directly 'on the inputimage wall 15 .of envelope 11 andthe transparent support plateV 27-omitted. VO11 `the. electron gun side of the transparentY support plate27 there .is providedatransparent conductive coating 34) which may l.beany .well known light transparent, electrically conductive material suchas tin chloride or .tin oxide. On the surjface of the transparentconductivercoating 30 is provided a layer, of approximately l() milsthickness, of `photoconductivev material 32, such as cadmium sulphidepow- Ader. .On the exposed surface Vof photoconductive layer '32 thereis provided an opaque resistive layer 34 that may be .formed of amaterial suchas conducting cadmium sul,- phide, or carbon particlesinanplastic binder, and may Vlbe approximately one to two mils thick. Onthe opaque Vresistive layer 34 there is provided a layer of phosphormaterial 36 that may be of .a material such aszinc oxide. Supportedwithin the envelope 11and closely adjacent to the target 25, `is la`conducting fine mesh screen '3.8 o f anyV high transparency. The meshscreen 38 is supported over an 'open end o'f a hollow tubular electrodeV39,. 'fIhe velectrode 39 .may take the form of a conductive coatingapplied to the inner surface of envelope 11 and screen A38 may besupported by electrodes extending through 'the walls of the envelope.The photoconductive llayer 32 is a resistive layer that has .a lowconductivity inthe dark, and a high conductivity when light is focusedthercon.r

Since, during operation, there is a potential drop across .the.photoconductor 32, the photoconductive layer 32 should be Vthick enoughto sustain a potential drop 'inthe order of 500 volts withoutVbreakdown. Due to Ythis f limitation, it .may bedesirable to .utilizeaphotoconductor of the powdered form such as `cadmium selenide and cad'-miumV sulphide, either of which may be made intoa thick layer, as thephotoconductive layer 32. .It should be .understood that-photoconductors that are deposited Vb y evaporating material, egg.'antimony trisulphide, may also be utilized, andthese layersmade thickenoughto adequatelysustainthe voltage drop across the photoconductor.

"When it is.desred to visiblyV reproduce `infrared images, aphotoconductive material should be .usedfor -the photoconductivelayer 32that is sensitive in the infrared range of the spectrum. One .example ofsuch a photoconductor is'lead sulphidef- When it isjdesired to visiblyreproduce .X-ray images, Ya photoconductor, such as cadmium sulphide,should be utilized whichresponds well to frequencies ,inV this range ofthe spectrum. i

"The purpose of the opaquelayer 34 in the target 225 .is tol prevent"light 'feedback 'from the phosphor :36"toithe photoconductive layer 32.'In otherwords, Vit is to prevent vprevent loss o'fepicture resolution.This normally'results linthe opaquelayer "3'4 being relativelythin. The'apague' resistive layer "34 may 4be -formedof a mosaic, 'oraptub ralityof opaque conductive islands such as evaporated aluminum islands (notshown) that are spaced apart so that the lateral conductivity across thetarget 25 is extremely low, but the conduction through the islands, isrelatively high. The opaque layer 34 may also be formed of knownconductive materials such as carbon particles in a suitable bindingmaterial.

The phosphor layer 36 should be relatively conductive, or be very thin,so that the potential on the exposed surface of the phosphor layer 36Will be substantially the same as the potential on the gun side of thephotoconductive layer 32. In other words, when in the dark, the majorityof the voltage drop across an elemental crosssectional unit of thetarget should occur across the photoconductive layer 32 rather thanacross the phosphor layer. One example of a highly conductive phosphoris zinc oxide.

During one form of operation, and using the potentials shown in Figure 1as an example, a potential of 500 volts positive with respect to thegrounded cathode Z1 is applied to the transparent electrode 30. With theelectron beam turned off, the phosphor surface 36 acquires a potentialof 500 volts by leakage to the electrode 30. When the electron beam isturned on, the phosphor surface is driven in a negative direction untilthe electrons in the beam are equal to the charge leakage through thetarget, i.e. the leakage from the scanned surface of phosphor 36 to thetransparent conductor 30. By adjusting the density of the electron beam,the potential of the surface of the phosphor may be stabilized at a fewvolts positive, e.g. tive to ten volts, with respect to the cathode 21.At this point a current equal to the primary electron beam current willow through the series connection of the phosphor layer, the opaquelayer, and the photoconductive layer to the electrode 30. Due to theresistance of these layers, a certain voltage drop, of approximately 495volts, is established across the target in the dark. When the phosphoris at this stabilized potential, the landing velocity of the electronsfrom gun 19 onto phosphor 36 is very low and no light is producedthereby when the electrons land on the phosphor 36 to replace the chargeconducted through the target. Thus, as long as the photoconductive layeris maintained in the dark, its resistance remains high and the primaryelectron current owing through it from the gun 19 will build up arelatively large Voltage vdrop across it.

When an image is directed onto the photoconductor 32, the resistance ofthe photoconductor is lowered in the areas struck by light from theimage. This decrease in resistance, decreases the voltage drop throughthe target and thus increases the potential of the phosphor with respectto the cathode 21. The electrons now land at higher velocities in theseareas and produce visible images. These images vary in accordance withthe input signals.

The device of Fig. 1 may also be constructed using a phosphor which isrelatively insulating. If such a phosphor is used, a target as shown inFig. 2 may be employed. This target comprises a transparent supportplate 41 having a transparent conductive coating 43 on one surfacethereof. On the transparent conductive coating 43 is a layer ofphotoconductive material 46 which in turn supports an opaque resistivelayer 47. .On the opaque resistive layer 47 is a phosphor layer. 48 thatcomprises a mosaic of phosphor areas 48 with the resistive layer 47exposed between the phosphor areas 48. In this arrangement, the phosphorareas 48 assume approximately the potential of the exposed opaque layer47, which acts as a collector electrode (assuming that more secondaryelectrons are dislodged from the target than primary electrons land onthe target). Since no secondary electrons from the phosphor areas canreach screen 38, because of the suppressing eld, the net currententering the opaque layer 47 is equal to the priymary current, and theoperation will be similar to that described for the target of Figure 1.The materials for the target shown in Figure 2 may be similar to thosedescribed in connection with Figure l.

Referring now to Figure 3 there is shown a cross sectional view of anembodiment of this invention comprising an evacuated envelope 52 havingan electron gun 54 in one end of the envelope for producing an electronbeam 56. The electron gun 54 may be any conventional type that isadapted to be scanned over the surface of a target by any continualmeans such as horizontal and vertical deflection plates 53. The electronbeam 56 is directed toward the target 5S that comprises a phosphor layer60 that is supported in light exchange relationship with the balance ofthe target 58. The balance of the target 58 includes a transparentsupport plate 59, a transparent conductive coating 62, a photoconductivelayer 64, an opaque resistive layer 66 and a phosphor layer 68. Thematerials for the layers of the target 58 may be substantially the sameas those previously described with the phosphor layer 60 being of amaterial similar to the phosphor 36 described above. In this embodimentof the invention, the electron beam 56 is video modulated to produce avisible picture on the phosphor 60. This picture is intensified by thephotoconductor 64, and the phosphor 68, similar to that described inconnection with Figure 1.

In any of the embodiments of this invention, the opaque resistive layermay be omitted, and the device used to store a transverse picture forany desired length of time. The storage occurs because now thephotocon'- ductor is in light exchange relationship with respect to thephosphor, which produces a regenerative feedback action that willcontinuously energize elemental areas of photoconductor and phosphorwhich have been triggered on.

The device in accordance with this invention may be scanned by anelectron beam, rather than sprayed with an electron stream, when it isdesired -to increase the instantaneous current density in the device.

Image intensifying devices in accordance with this invention are verysensitive, and utilize the relatively high sensitivity ofphotoconductive materials. Also, these devices do not have the problemsof electron optically focusing imaging currents.

What is claimed is:

l. An image intensifying device, comprising an evacuated envelope, asource of electrons within said envelope, a target electrode in saidenvelope and in the path of said electrons, said target electrodeincluding4 a transparent conductive electrode, a layer ofphotoconductive material on said conductive electrode, an opaqueresistive layer on said layer of photoconductive material and a layer ofphosphor material on said opaque resistive layer, said layer of phosphormaterial being exposed and on the surface of said target toward saidsource of electrons. y

2. An `image intensifying device, comprising an evacuated envelope, anelectron source Within said envelope, a tar-get electrode in the path ofsaid electrons and including a transparent conductor, a layer ofphotoconductive material on said transparent conductor, an oapqueresistive layer on said photoconductive material, a mosaic of phosphoron said opaque resistive layer, elements of said phosphor mosaic beingexposed and spaced apart on the surface of said opaque resistive layerin the path of said electrons.

3. An image intensifying device comprising an evacuated envelope, atarget electrode within said envelope and including a transparentsupport member, a rst layer of phosphor material on one side of saidsupport member, a transparent conductive coating on the other surface ofsaid support member, a layer of photo-conductive material on saidtransparent conductive coating, an opaque resistive coating on saidphotoconductive material, and a second layer of phosphor material onsaid opaque coating, means within said envelope for producing anelectron beam, means for scanning said beam over said rst layer ofphosphor material, means within said envelope for producing a stream ofelectrons, and means for directing said stream of electrons onto saidsecond phosphor layer.

4. An image intensifying device, comprising an evacuated envelope, atarget within said envelope, said target including a rst phosphormaterial, a photoconductor and a second phosphor material, means Withinsaid er1-4 velope for producing an electron beam, means for scaning, anopaque resistive coating on said photoconduc-l tive material and asecond phosphor material on said resistive coating, au electron lgunWithin said envelope for producing an electron beam, means VWithinsaidenvelope for scanning said beam over said first phosphor material t atenergies that produce light, the resistance of saidk ning said beam oversaid first phosphor material, a Y

source of electrons within said envelope, means for directing electronsfrom said source onto said second phosphor material, and said secondphosphor material and said photoconductor forming an electrical seriespath whereby the electrons flowing through said photoconductor originateat said source.

5. An image intensifying device, comprising an evacuated envelope, atarget electrode within said envelope and including a transparentsupport member, a irst layer of phosphor material on one side of saidsupport member, a transparent conductive coating on the other surface ofsaid support member, a` layer of photocon- Y ductive material on saidtransparent conductive Vcoatl Re. 23,802

' photoconductor being decreased by said light, an electron gun withinsaid envelope for producing a stream of electrons, and kmeans fordirecting said stream of eleotronsvonto said second phosphor material.

References Cited in the le of this patent K UNITED STATES PATENTS l1,954 iVV

